This invention relates to a time division communication system, and more particularly to a burst signal generating system where generation of an undesired signal is suppressed, when the burst signal is generated, by switching a modulated wave under a band limitation in a baseband.
In a time division communication system, a modulated signal, such as a phase modulated or amplitude modulated signal, is transmitted in the form of a burst signal because such a signal is transmitted in an assigned time slot. Therefore, in such a time division communication system, a switch is usually provided after a modulator and a burst signal is generated by switching the modulated wave using the ON/OFF control of a rectangular signal. Such a burst signal is then inserted into the assigned time slot.
However, when such switching is carried out for the modulated wave by a switch, an undesired signal is generated because of an impulse response caused by the front and rear edges of the burst signal. The undesired signal is generated on the basis of the impulse of response of the system. Consequently, a spectrum generated by the impulse is wide, influencing other communication channels in the system. For example, a plurality of time dividing channels operating at different frequencies co-exist in a time dividing system, and in such a case, a widely spreading spectrum may adversely influence the other time dividing channels.
The impulse spectrum distribution can easily be limited by using a filter and the undesired signal can be eliminated by connecting a bandpass-filter with a narrow frequency pass band to the succeeding stage of the switch circuit.
However, this method is, of course, not capable of eliminating the undesired signal component within the pass band of the bandpass-filter and, moreover, it is difficult, as is well known, to construct a filter which provides a very narrow band limitation in an RF (radio frequency) signal.
Thus, it is necessary, in one method used, for proper band limitation, to perform such band limitation in an intermediate frequency and then convert the intermediate frequency up to the radio frequency. But this method also has a problem. Namely, when a bit rate is high as compared to the RF carrier frequency, an undesired signal can be eliminated easily because the main lobe is in the vicinity of the RF carrier frequency. Therefore, it is necessary to execute a band limitation in the baseband frequency and then convert the frequency up to the radio frequency by a heterodyne method.
Moreover, an undesired signal resulting from the switching operation can easily be eliminated by such a simple method in which the rectangular wave signal for controlling the ON/OFF of the burst signal is smoothed. However, it is difficult for this method to shape the waveform at the rising and falling portions because a diode is usually employed as the switching element and, in some cases, data is lost, level dropping occurs, and burst interference occurs at the extremely gentle rising and falling portions of the waveform.